You can't definatley tell which account is and alt and who it belongs to
these is because of IP being shared by people in the same house, place, work, institution, ect. and then there is a problem with private IP, that some browsers used to offer their services. Now imagine that more than 20 people in that same place, work ect use the same server/PC to play. So unless you have a picture of person A login in at the account of person B, and person A can't prove that person B is real then you can't do naught.
but even if person you have such a picture, there person A can still argue that he/she is holding/managing/using the account for person B, on person B agreement/approval.
So the problem is not shutting alts down, is as you say ''Find out and get to know is alt of whom''
league generally have and intelligent agency that handle such trivial things, they monitor boring things like log in time, chat activity interaction activity on cattle ect. using these infor they can tell where the person come from, what time he plays ect, they use these info and compare it with the info they get from their spies which they send out in advance to their enemy site. It will narrow down stuff
some BIG league use moderators, who get IP, and then compare it, to see whose alt is whose, since moderators can see players IP. these is why every top league make sure they have 1 or 2 moderators. We have seen these tactic been used not long ago here on the forums.
Some leagues used VIP connections to bribe/ask/inquire about alts, since VIP are in close contact whith plarium stuff, IP can be compared easily from there, though I am not sure if these method is used, but you never know
I use Raid/offence/BG bar on profile information, and the friend list to tell if a person is an alt.
Hope these help, before it gets delete